AQA A-Level Biology (7402)

## Learning Outcomes

- Define population, community, habitat and niche accurately
- Explain how abiotic and biotic factors affect population size and distribution
- Select suitable methods for estimating abundance in sessile and motile organisms
- Calculate population size using mark-release-recapture and biodiversity using Simpson’s index
- Explain primary, secondary and deflected succession step by step
- Evaluate ecological data and methods using precise biological reasoning

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand populations in habitatss, with a focus on the following syllabus points:

- Know the meaning of population, community, habitat and niche
- Understand how abiotic factors and biotic factors affect the distribution and abundance of organisms
- Identify appropriate methods for sampling populations, including random quadrats, transects and mark-release-recapture
- Calculate population size from capture-recapture data and interpret the assumptions behind the estimate
- Use Simpson’s index of diversity to compare biodiversity between habitats
- Understand succession as a change in communities over time, including the role of pioneer species and the development of a climax community
- Explain how organisms change their environment during succession and how this affects later colonisers
- Distinguish between primary, secondary and deflected succession

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between a population, a community, a habitat and a niche?
2. Why are random quadrats suitable for some species, but mark-release-recapture is needed for others?
3. How do abiotic factors such as temperature, light and water availability change carrying capacity?
4. What assumptions must be true for mark-release-recapture to give a reliable estimate?
5. How does succession cause biodiversity to change over time?

## Introduction

Populations in habitatss is a high-value synoptic topic because it joins together ecology, evolution, practical methods and mathematical skills. In exams, students often know the definitions but lose marks when they have to explain why a method is used, why a result may be unreliable, or how environmental change alters species distribution.

This topic is really about three linked questions: where organisms live, why they live there, and how biologists measure that pattern. To answer those well, you need precise definitions, a secure grasp of sampling methods, and clear cause-and-effect chains for succession and biodiversity.

> **Key Term: environment**
> A dynamic system made up of a community of organisms and the abiotic factors of its environment.

### Core ecological relationships and why populations change

A **population** is all the organisms of one species in a particular area at a particular time. A **community** is all the populations of different species living together in that area. A **habitat** is the place where an organism lives, while its **niche** is its role within that habitat.

Students often mix up habitat and niche. Habitat is the address; niche is the job. Two species may share a habitat but not the same niche. For example, two plant species in the same grassland may absorb mineral ions from different soil depths or flower at different times. That reduces direct competition.

> **Key Term: population**
> All the organisms of one species living in the same area at the same time and potentially able to interbreed.
>
> **Key Term: community**
> All the populations of different species living and interacting in one area.
>
> **Key Term: niche**
> The role of a species within its habitat, including how it uses resources and interacts with other organisms.

Population size is not fixed. It changes because of four processes:

- **natality** increases numbers
- **immigration** increases numbers
- **mortality** decreases numbers
- **emigration** decreases numbers

A population can only grow if conditions allow enough individuals to survive and reproduce. The largest population size the environment can support is the **carrying capacity**. This is not a permanent number. It changes if conditions change.

Abiotic factors affect carrying capacity because they change survival, growth and reproduction. Temperature is a good example. In plants and ectothermic animals, low temperature slows enzyme-controlled reactions, so metabolism falls, growth slows and fewer offspring are produced. At high temperatures, enzymes may denature, again reducing growth and reproduction. In both cases, carrying capacity falls.

Warm-blooded animals are different, but not exempt. If external temperature is far from their optimum, they use more energy on thermoregulation. Less energy is left for growth and reproduction, so carrying capacity can still decrease.

Other major abiotic factors include:

- light intensity
- pH
- water availability
- humidity

Each affects enzyme action or water balance. That is why some species are common only in certain conditions. A cactus can be abundant in dry habitats because it is adapted to low water availability, while less drought-tolerant species cannot survive there.

Biotic factors also matter. These include:

- intraspecific competition
- interspecific competition
- predation
- disease

Intraspecific competition is often strong because members of the same species need almost identical resources. If more individuals survive, competition for food, light, territory or mates rises. This can reduce survival and slow population growth.

Predator-prey cycles are a classic example of linked populations. If prey numbers rise, predators have more food, so predator numbers may increase after a time lag. Increased predation then reduces prey numbers. With fewer prey available, predator numbers later fall, allowing prey numbers to rise again.

> **Key Term: carrying capacity**
> The maximum population size that an environment can support at a given time.
>
> **Test Tip:** For 2- to 4-mark explanation questions, do not just list factors. Link factor to physiology, then to survival or reproduction, then to population size.

### Sampling populations and measuring abundance

Ecologists usually cannot count every organism in a habitat, so they estimate abundance by sampling. The method depends on whether organisms are sessile, slow-moving or motile, and whether the habitat is fairly uniform or changing along a gradient.

> **Key Term: abundance**
> The number of individuals of a species in a given area.

For plants and other sessile organisms, quadrats are often used. A quadrat is a frame of known area placed on the ground so organisms inside it can be recorded. If the habitat is fairly uniform and you want an estimate for the whole area, quadrats should be placed randomly.

Random sampling matters because non-random placement creates bias. If you place quadrats where plants look most obvious, abundance will be overestimated. A proper method is to lay two tape measures at right angles, generate random coordinates, and place the quadrat at each chosen point.

There are several ways to measure abundance using quadrats.

**Frequency** records whether a species is present in each quadrat.

$$
\text{frequency} = \frac{\text{number of quadrats containing the species}}{\text{total number of quadrats}}
$$

Percentage frequency is this value multiplied by 100.

Frequency is useful when a species is difficult to count individually, but it does not tell you how many organisms are present in each quadrat. A species found once in every quadrat and a species covering most of every quadrat could have the same frequency.

**Percentage cover** estimates the percentage of the quadrat area occupied by a species. This is useful for plants such as grasses, mosses or clover, where individual organisms may be hard to separate.

$$
\text{percentage cover} = \frac{\text{area covered by species}}{\text{total quadrat area}} \times 100
$$

This is often better than counting individuals, but it is more subjective because different observers may estimate slightly different values.

> **Key Term: random sampling**
> Sampling in which every location has an equal chance of being selected, reducing bias.
>
> **Key Term: percentage cover**
> An estimate of the proportion of the area in a quadrat occupied by a species.

If there is a clear environmental gradient, such as from seashore inland, random quadrats alone are not enough. You need a transect.

A **line transect** records the species touching a tape or line. A **belt transect** uses quadrats placed at intervals along a line, so it gives information about both distribution and abundance across the gradient.

This is especially useful in Required Practical 12. For example, if you investigate the effect of soil moisture on a plant species, you could place quadrats every metre along a transect from wet ground to drier ground. At each point, record the species abundance and measure the abiotic factor at the same point. That matching is essential. If abundance and soil moisture are measured in different places, the relationship becomes weak or meaningless.

A common exam trap is to claim that correlation proves causation. It does not. If plant abundance increases with soil pH, pH may be important, but another linked factor could also be involved, such as mineral availability, competition or drainage.

> **Test Tip:** When evaluating a fieldwork method, high-mark phrases include larger sample size, repeat measurements, representative sample, suitable quadrat size, reduced observer bias, and correlation does not prove causation.

For motile organisms, quadrats are unsuitable because the organisms move away. Here, mark-release-recapture is used.

### Mark-release-recapture and biodiversity calculations

In mark-release-recapture, a first sample is captured, counted, marked harmlessly and released. Later, a second sample is captured. Some individuals in that second sample will already be marked. If many are marked, the total population is probably small. If few are marked, the population is probably large.

The estimate is given by the Lincoln index:

$$
N = \frac{n_1 \times n_2}{m_2}
$$

Where:

- $N$ = estimated population size
- $n_1$ = number captured and marked in the first sample
- $n_2$ = total captured in the second sample
- $m_2$ = number of marked individuals recaptured

Worked example:

A student catches 40 woodlice, marks them and releases them. Two days later, the student catches 50 woodlice. Of these, 10 are marked.

Step 1: write the values clearly

- $n_1 = 40$
- $n_2 = 50$
- $m_2 = 10$

Step 2: substitute into the equation

$$
N = \frac{40 \times 50}{10}
$$

Step 3: calculate

$$
N = \frac{2000}{10} = 200
$$

Estimated population size = **200 woodlice**

This is an estimate, not an exact count.

> **Key Term: mark-release-recapture**
> A method for estimating the population size of a motile species by comparing the proportion of marked individuals in a second sample.

For this estimate to be reliable, several assumptions must hold:

- the population is closed, with no major immigration or emigration
- few births or deaths occur between samples
- marked individuals mix fully back into the population
- the mark does not affect survival
- the mark does not affect chance of recapture
- the mark is not lost

These assumptions are often turned into exam questions about error. For example, if the mark makes animals more visible to predators, marked individuals are removed faster. Fewer marked animals are recaptured, so $m_2$ is smaller. Because $m_2$ is on the bottom of the equation, the estimated population size becomes too high.

If lots of births occur between samples, there are more unmarked individuals in the second sample, so again $m_2$ becomes proportionally smaller and the estimate becomes too high.

Biodiversity is measured not just by species richness, but also by evenness. A habitat with five species present in similar numbers is more diverse than a habitat where one species dominates and the other four are rare.

AQA uses Simpson’s index of diversity:

$$
D = 1 - \sum \left(\frac{n}{N}\right)^2
$$

Where:

- $n$ = number of individuals of one species
- $N$ = total number of individuals of all species

A higher value of $D$ means greater biodiversity.

Worked example:

A meadow contains:

- species A = 20 individuals
- species B = 15 individuals
- species C = 5 individuals

Step 1: find $N$

$$
N = 20 + 15 + 5 = 40
$$

Step 2: calculate each $(n/N)^2$

$$
\left(\frac{20}{40}\right)^2 = 0.25
$$

$$
\left(\frac{15}{40}\right)^2 = 0.140625
$$

$$
\left(\frac{5}{40}\right)^2 = 0.015625
$$

Step 3: add them

$$
\sum \left(\frac{n}{N}\right)^2 = 0.25 + 0.140625 + 0.015625 = 0.40625
$$

Step 4: subtract from 1

$$
D = 1 - 0.40625 = 0.59375
$$

So Simpson’s index of diversity is **0.594** to 3 significant figures.

Students often make three errors here:

- forgetting to square each fraction
- using species number instead of individual number
- treating the index as meaningful on its own rather than mainly for comparison
  > **Key Term: Simpson’s index of diversity**
  > A measure of biodiversity that takes account of both species richness and the relative abundance of each species.

### Succession and changing communities over time

Succession is the change in species composition of a community over time. The key idea is that organisms do not just live in an environment; they also change it. Those changes may make conditions more suitable for new species and less suitable for earlier ones.

> **Key Term: succession**
> The change in community composition over time as different species colonise and alter a habitat.

In **primary succession**, colonisation begins on a bare surface with no soil, such as bare rock. The first colonisers are pioneer species, often lichens or mosses. These species can survive harsh conditions because they are tolerant of desiccation, low nutrient levels and temperature fluctuations.

Pioneer species change the environment in several ways:

- they weather the rock
- when they die, decomposition adds organic matter
- humus begins to form
- soil depth increases
- mineral ion availability rises
- water retention improves

These changes make the habitat less hostile. New plants can establish, often grasses first, then shrubs, then larger plants. As vegetation height and complexity increase, more animals can live there because there are more food sources and more niches.

Over time, biodiversity usually increases. Food webs become more complex and the environment becomes more stable. Eventually, a relatively stable **climax community** may form, determined largely by climate and soil conditions.

In **secondary succession**, a community has been disturbed but soil remains. Recovery is faster because seeds, spores, nutrients and microorganisms are already present.

In **deflected succession**, human activity prevents succession reaching the climatic climax community. Grazing is a common example. If sheep continually eat young tree seedlings, woodland may never develop, and the habitat remains grassland.

The important exam logic is this: succession is not just a list of stages. You must explain how one stage causes the next. For example, pioneer species increase organic matter, which improves water retention, which allows less drought-tolerant species to survive, which then outcompete pioneers in the changed conditions.

A common pattern during succession is:

- increased soil depth
- increased humus
- increased mineral ions
- improved water retention
- increased biodiversity
- increased biomass

Ground-level light often decreases later because taller plants shade the surface. That is why some early species disappear.

> **Key Term: pioneer species**
> The first species to colonise a bare or disturbed habitat, usually adapted to harsh conditions.
>
> **Key Term: climax community**
> The relatively stable community present at the end of succession.
>
> **Test Tip:** In succession questions, use arrows or clear linking words: pioneer species die -> decomposition adds humus -> soil depth increases -> water retention improves -> larger plants establish. This gains marks for mechanism.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A population is all the organisms of one species in an area at one time, while a community contains all populations of different species in that area
- A habitat is where an organism lives, but a niche is its role and interactions within that habitat
- Population size changes through natality, mortality, immigration and emigration
- Carrying capacity depends on abiotic conditions and biotic interactions
- Random quadrats are used to estimate abundance of sessile or slow-moving organisms in a fairly uniform area
- Belt transects are used to study changes in distribution and abundance along an environmental gradient
- Frequency records how often a species occurs in quadrats, while percentage cover estimates how much area it occupies
- Mark-release-recapture is used for motile organisms and depends on assumptions such as a closed population and harmless marking
- The Lincoln index estimates population size using first sample size, second sample size and number recaptured
- Simpson’s index of diversity measures biodiversity using both species richness and relative abundance
- Succession is the change in community composition over time, usually leading to increased soil development, biomass and biodiversity
- Primary succession starts without soil, secondary succession starts where soil remains, and deflected succession is held back by human activity

## Key Terms and Concepts

- environment
- population
- community
- niche
- carrying capacity
- abundance
- random sampling
- percentage cover
- mark-release-recapture
- Simpson’s index of diversity
- succession
- pioneer species
- climax community


Genetics, populations, evolution and ecosystems - Populations in ecosystems